Patent Documents 1, 2 disclose vehicle-height control systems each including: an air supply device including a tank, a compressor device, and a cut-off valve provided between the tank and the compressor device; and air cylinders each as a vehicle-height control actuator. In these vehicle-height control systems, air is supplied from the tank or the compressor to the air cylinders to execute up control for increasing a vehicle height.
In the vehicle-height control system disclosed in Patent Document 1, a discharge portion of the compressor and the air cylinders are connected by a first passage and a second passage in parallel, and the cut-off valve as an electromagnetic valve and the tank are provided in series on the first passage. In the vehicle-height control system disclosed in Patent Document 1, in the case where a tank pressure that is a pressure of air contained in the tank at a timing just before the up control (noted that this timing may be hereinafter referred to as “at start of the up control”) is greater than or equal to a set pressure, a tank-pressure supply state is established in which air is supplied from the tank to the air cylinders via the first passage. In the case where the tank pressure at the timing just before the up control is less than the set pressure, the compressor device is actuated, and a compressor-pressure supply state is established in which air is supplied from the compressor to the air cylinders via the second passage. The cut-off valve is kept closed during the up control.
In the vehicle-height control system disclosed in Patent Document 2, the tank is connected via an electromagnetic valve in the form of the cut-off valve to a portion of a passage that connects the discharge portion of the compressor and the air cylinders to each other. In the vehicle-height control system disclosed in Patent Document 2, a set length of time is determined based on a difference between a tank pressure and a pressure of the air contained in the air cylinders at a timing just before the up control. When the up control is started, the tank-pressure supply state is established by opening of the cut-off valve. However, when the set length of time is elapsed, the cut-off valve is closed, and the compressor device is actuated to switch a state to the compressor-pressure supply state.